The invention relates to a process for the continuous preparation of aqueous ammonium thiocyanate solutions by reaction of ammonia with carbon disulfide in the presence of water using activated carbon as catalyst.
Thiocyanates (rhodanides) in the form of their alkali or ammonium salts are chemicals that not only are frequency used in chemical laboratories, but are also extremely important in other technical fields. Thus, for example, an aqueous thiocyanate solution is frequently used as a precipitation bath during the wet spinning of polyacrylonitrile. Thiocyanates can also be employed for the preparation of polyacrylonitrile solutions. Thiocyanates used for this purpose have high requirements.
Some processers for the preparation of thiocyanates consist, first, in preparing an aqueous solution of ammonium thiocyanate. This solution can be processed further into solid ammonium thiocyanate likewise in a manner known per se or reacted with the hydroxide of an alkali metal to alkali-metal thiocyanate.
The preparation of ammonium thiocyanate and aqueous ammonium thiocyanate solutions by reaction of ammonia with carbon disulfide in the presence of water has long been known in the art. Thus U.S. Pat. No. 2,249,962 describes the reaction of ammonia with carbon disulfide in a boiler in the presence of water and a catalyst. Fatty acids, especially oleic acid or ammonium fatty acid salts, tall oil, or high alcohols are used as the catalyst. The reaction is carried out in the presence of alcohols a solubilizers. One of the disadvantages of this process is the presence of the solubilizer, which must be removed after the reaction is completed. The catalysts employed cause problems during the processing of the reaction mixture. Therefore, the separation of the undesirable constitutents and the purification of the end product are very costly. Moreover, the catalyst must be renewed frequently. In addition, sulfur forms during the reaction, which likewise leads to processing problems. Since the reaction is carried out in several stages at various temperatures, the process technology is complicated.
U.S. Pat. No. 2,850,356 describes the reaction of ammonia and carbon disulfide in a reactor which consists of three zones and is provided with packing material, e.g., Raschig rings. Although this patent, too, mentions that processes for the reaction of ammonia with carbon disulfide under pressure are known, it is recommended that the process be carried out at standard pressure, if necessary at a slightly elevated pressure up to about 1.7 bar. The reaction is carried out in such a way that temperatures of 35.degree., 55.degree. and 80.degree. C. are established in the three stages (zones). The process does not use a catalyst.
Accepted West German Patent Application No. 1,297,088 describes a process in which activated carbon is used as a catalyst. Here, too, the reaction is carried out in three stages. Accepted West German Patent Application No. 1,592,341 discloses an improvement of the process disclosed in Accepted West German Patent Application No. 1,297,088, and describes the reaction of gaseous carbon disulfide at temperatures of from 70.degree. to 90.degree. C.
The processes described in the last two patents already lead to high reaction rates and high yields of ammonium thiocyanate with purities exceeding 99%.
Ever-increasing requirements for the quality of ammonium thiocyanate and the efficiency and cost-effectiveness of the manufacturing process, however, lead to the demand for other processes that would result in higher efficiency and greater economies.